CDS dumping uses ResourceHashtable, for example, to store the relocated address of every archived MetaspaceObj. See

http://hg.openjdk.java.net/jdk/jdk/file/f1bb77833b59/src/hotspot/share/memory/metaspaceShared.cpp#l1087

  typedef ResourceHashtable<
      address, address,
      ArchiveCompactor::my_hash,
      ArchiveCompactor::my_equals,
      16384, ResourceObj::C_HEAP> RelocationTable;

However, the template class ResourceHashtable requires the size to be statically specified in the source code. Currently we pick the size 16384 because it's not too big and it is sufficient for up to a few thousand classes.

However, if we dump all the classes in the system modules (about 35000 classes), the RelocationTable would need to store about 2,000,000 records, making each bucket over 100 entries. This causes drastic slow down with CDS dumping.

Test case -- LotsOfClasses.classlist is in attachment

 java \
    -XX:MaxRAM=8g -Xshare:dump \
    -Xlog:cds,cds+hashtables -Xlog:hashtables \
    -XX:SharedArchiveFile=LotsOfClasses.jsa \
    -XX:ExtraSharedClassListFile=LotsOfClasses.classlist \
    --add-modules ALL-SYSTEM

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First draft: http://cr.openjdk.java.net/~iklam/jdk12/8213587-configurable-resource-hash.v01/

BEFORE: 93.971 sec
AFTER: 34.761 sec